Photodetectors capable of detecting a single photon (i.e., a single “particle” of optical energy) are useful in many applications. To date, most of these applications have relied on the use of single-photon detectors such as photomultiplier tubes (PMTs) or single-photon avalanche detectors (SPADs) that are silicon-based, and are therefore capable of efficiently detecting only photons that have a wavelength within the range of approximately 250 nanometers (nm) to approximately 900 nm. New applications are emerging, however, that require single-photon detectors that can operate at high speed (>1 MHz) and at longer wavelengths (>1000 nm). Such devices would find use in areas such as: quantum information processing, quantum computing, quantum cryptography, and quantum teleportation and communications; low-light-level imaging and other high-performance imaging applications; and others. Unfortunately, currently available SPADs do not have the combination of high operational speed and wavelength range required for many of these applications.
An avalanche photodiode (APD) is a type of photodetector that is capable of providing extremely high sensitivity. An APD derives its name from the manner in which its output signal is created. When an APD absorbs photons, their energy excites electrons normally bound in the atomic lattice of the APD material to create free electrons. Each freed electron leaves behind a positively charged vacancy (i.e., a “hole”) in the crystal structure. These electrons and holes are free-charge carriers that can flow freely through the structure of the APD.
In the presence of an electric field (due to a bias voltage applied across the APD), these free-charge carriers are accelerated through a region of the avalanche photodiode known as the “multiplication region.” As the free-charge carriers travel through the multiplication region, they collide with other electrons and holes bound in the atomic lattice, thereby generating more free-charge carriers through a process called “impact ionization.” These new free-charge carriers also become accelerated by the applied electric field and generate yet more free-charge carriers.
When operated in “Geiger mode,” an APD can be made sensitive enough to detect even a single photon, and a device designed specifically for this purpose is referred to as a single-photon avalanche diode (SPAD). In Geiger-mode operation, a SPAD is “armed” by biasing it with a voltage that is above its breakdown voltage, which is the voltage bias level above which free-charge carrier generation can become self-sustaining and result in a run-away avalanche. Arming a SPAD puts it in a meta-stable state in which absorption of a single photon can give rise to a runaway avalanche that results in an easily detectable macroscopic current. This avalanche event can occur very rapidly and efficiently and it is possible to generate several hundred million free-carriers from a single absorbed photon in less than one nanosecond (ns).
In order to prepare the SPAD for re-arming once this current is generated, the avalanche current must be halted. This is done with a process referred to as “quenching,” wherein the bias voltage is reduced to a value sufficiently close to the breakdown voltage that the avalanche can spontaneously terminate.
Controlling voltage bias to arm and quench an APD is one of the primary challenges for Geiger-mode operation and the rate at which a single-photon detector can be operated is determined by (1) how quickly the APD can be quenched once a photon has been detected and (2) how quickly the APD can be re-armed once it has been quenched.
Although quenching stops the avalanche process, not all free carriers are instantaneously swept out of the avalanche region. Instead, some carriers become trapped in the multiplication region in trap energy states, which arise from crystalline defects or other causes. These trapped carriers are released in a temporally random manner based on such factors as temperature, the type of trap state, and the applied bias voltage. When a trapped carrier is released after the SPAD has already been re-armed, there is a possibility that it can initiate impact ionization as if the APD has absorbed a photon. As a result, the detrapping of a carrier can result in a “false” electrical signal that occurs in the absence of photon absorption. A false count that occurs in the absence of a photon absorption is referred to as a “dark count,” and dark counts that arise specifically from detrapping of trapped carriers are referred to as “afterpulses.”
The temporal variation in the rate of dark counts constitutes noise in a single-photon avalanche detector. As a result, afterpulses degrade SPAD sensitivity. One approach for improving sensitivity in the presence of afterpulsing is to simply delay rearming after quenching. This allows trapped charges a sufficient period of time to detrap while the SPAD remains unarmed. Unfortunately, such an approach requires an undesirably long period of time when the single-photon detector is insensitive to incident photons.
Alternative approaches for reducing afterpulse effects include 1) actively inducing rapid detrapping of trapped charges; 2) stifling the detrapping of trapped charges; and 3) limiting the number of free carriers that flow through the multiplication region during an avalanche event.
Actively induced detrapping can be accomplished in several different ways, such as heating the photodiode or energizing the carriers by illuminating them with light at a different wavelength. Such approaches, however, have shown very limited success. Elevating the temperature of an APD imposes a severe tradeoff by increasing the dark count rate while sub-bandgap illumination has not yet been shown to effectively induce carrier detrapping. In addition, these approaches increase cost and complexity, making these approaches undesirable in many applications.
The stifling of trapped charges by lowering the temperature of a SPAD to “freeze” trapped charge carriers has not been successfully demonstrated. In fact, for practical SPAD devices, this approach is likely to increase after-pulsing as temperature is reduced. Further, if carrier freeze-out were successful, it is likely that at least some of the charge carriers associated with the dopant atoms would also be “frozen,” thus rendering the SPAD inoperable.
Some afterpulse reduction has been successfully demonstrated through the use of external circuitry to limit the flow of free carriers through the multiplication region during an avalanche event. However, the capacitance associated with the external circuitry adds to the RC time constant that dictates the rate at which a SPAD can be rearmed after quenching. For high-speed operation (i.e., >1 MHz), this RC time constant must be less than about 1 microsecond. As a result, any capacitance associated with external electronics that adds to the capacitance of the SPAD itself is generally undesirable. Moreover, the use of external circuitry generally involves significant complexity, and it can lead to additional undesirable parasitic elements in addition to the capacitance just described.
Monolithic, passive quenching approaches for limiting the flow of free carriers by have also been explored. The use of a monolithically integrated quenching (or feedback) load element can help to avoid the excessive parasitic capacitance inherent in the use of external circuitry. However, this legacy “negative feedback avalanche diode” (NFAD) is limited by an inherent tradeoff: a large feedback load is desired to promote rapid quenching, but a small feedback load is necessary to enable rapid re-arming. In prior-art NFADs, a single feedback load value is chosen to balance this tradeoff between the timescales for quenching and re-arming and, therefore, overall device performance is necessarily compromised.